A typical aircraft gas turbine engine combustor includes annular, radially spaced apart outer and inner combustion liners disposed coaxially about a longitudinal centerline axis to define an annular combustion zone therebetween. Disposed at the upstream ends of the liners is an annular combustor dome fixedly joined thereto and including a plurality of circumferentially spaced carburetors therein for providing a fuel/air mixture into the combustor which is ignited for generating combustion gases therein.
In order to protect the combustor from the hot combustion gases and provide a useful service life thereof, various means are used for cooling the dome and the liners including for example means for generating boundary layer films of cooling air along the inner surfaces of the combustor. Such film cooling must be provided from the dome and extend downstream along the full axial extent of the liners. The cooling air film is typically formed by a circumferentially extending conventional cooling nugget in the form of an annular plenum having a lip defining an annular outlet slot for discharging the cooling air film. The nugget includes a plurality of circumferentially space inlet holes typically located at a radius or apex of the nugget at an upstream end of the plenum. In conventional combustors, a plurality of axially spaced rows of the cooling nuggets are typically used for ensuring the continuation of an effective cooling air film along the entire axial and circumferential extent of the combustor liners.
Another type of combustor utilizes axially spaced rows of circumferentially spaced inclined multiholes for convectively cooling the liners as well as for providing the cooling air film along the inner surfaces thereof instead of using the conventional nuggets. However, in some multihole designs, the multiholes by themselves are unable to start an effective film of cooling air, and a conventional nugget or other means is provided for starting the cooling air film at the upstream end of the combustor immediately upstream of the first row of multiholes. An effective cooling air film in such an embodiment must be provided from the dome region of the combustor to ensure that the multiholes provide an effective cooling air film with an acceptable heat transfer rate. For example, one model test performed on behalf of the present assignee shows that the cooling effectiveness of a multihole liner alone as compared to a multihole liner having a cooling film starting slot at the upstream end thereof has initially about 10% of the cooling effectiveness compared thereto at the dome end of the combustor. The cooling effectiveness of the multihole-only liner increases along the liner in the downstream direction whereas the cooling effectiveness of the multihole liner with the cooling air film starting slot decreases in the downstream direction, with the film effectiveness of the latter being substantially greater than that of the former up to the last row of the multiholes in the liner.
The air used for providing a cooling air film in a combustor is a portion of compressor discharge air which necessarily decreases overall performance efficiency of the combustor since such air is not being directly used to support the combustion process. Furthermore, various conventional cooling air apertures exist in conventional combustors for providing cooling thereof including film cooling. Such various conventional structures have varying degrees of complexity, cost, undesirable weight, or stress concentrations, or efficiency of use of the available cooling air. For example, locating a cooling air aperture in the apex or radius of a plate typically results in a substantial stress concentration which must be accommodated, for example by increased thickness of the plate, for providing a useful life for the combustor. Furthermore, since combustor cooling air typically does not directly support the combustion process, the use thereof should be kept to a minimum to avoid overall performance efficiency losses of the combustor.